1. Field of the Invention
This invention relates generally to in-vehicle satellite navigation systems and, more particularly, to a method for estimating availability of signals from navigation satellites in which a host vehicle uses its own satellite signal quality information along with signal quality information from surrounding vehicles to create a sky visibility map.
2. Discussion of the Related Art
Many modern vehicles include navigation systems which use satellite-transmitted data to determine their latitude, longitude, and elevation—or similar location data in a different format, such as north and east offset with respect to a given origin. The Global Positioning System (GPS) in the United States is one example of these types of satellite navigation systems, collectively known as Global Navigation Satellite Systems (GNSS). Vehicles use the location data for embedded navigation purposes, and for other applications.
In order for a vehicle to determine its location using only GNSS, it must be able to receive signals from at least four satellites simultaneously. With additional aiding information such as height data provided using another system or sensor, this requirement may be reduced to three satellites or even fewer. There are normally several satellites which are theoretically visible to a vehicle at any given time, but some of the satellites may be at low apparent elevation angles, and thus may be obscured by some obstruction, such as a building or trees. Because the satellite signals follow a line of sight from the satellite to the receiver, these obstructions can diminish signal quality or completely block some signals. If some satellite signals are blocked, and a vehicle does not have good signals from four or more satellites, the vehicle will temporarily lose its location fix. This will cause a disruption in GNSS-based navigation capability, and will impact any other applications which use the location data.
There is an opportunity to share GNSS satellite data among a group of GNSS users who are in close proximity, such as in a vehicle-to-vehicle (V2V) network, to enable the construction of a sky visibility map. The sky visibility map would allow a vehicle to anticipate satellite signal availability, minimize the negative impact of satellite signal loss, and configure other vehicle systems to the current and approaching driving environment.